Auxin and growth regulation of rice coleoptile segments - molecular analysis

Auxin-stimulated elongation of apical segments of rice (Oryza sativa L. cv Arborio) coleoptiles occurring in the first 4 hours of treatment has been studied. Cell extension promoted in the first 2 hours by 10 micromolar indole-3-acetic acid (IAA) is specifically auxin-dependent, whereas after 4 hours, elongation also depends on endogenous production of ethylene. Similar to other systems, rice coleoptile cell elongation stimulated by auxin requires continuous synthesis of RNA and protein. Two-dimensional gel analysis of the in vitro translation products obtained from polyadenylated RNAs extracted from treated and untreated segments after 1 or 4 hours from the initial addition of IAA shows few transcriptional differences. At 60 minutes of treatment, the level of three mRNAs coding for proteins of 22.5, 25, and 33 kilodaltons was moderately enhanced while the disappearance of a 38 kilodalton translation product was observed. Additional repression of another mRNA coding for a 28 kilodalton product begins to show by this time, but becomes more evident after 4 hours treatment. At 4 hours, four IAA-specific mRNA enhancements coding for proteins with molecular masses ranging between 35 to 40 kilodaltons were also observed. We discuss these data in relation to the possible involvement of IAA-mediated transcriptional regulation in growth promotion of rice coleoptiles and, more widely, in control of cell elongation.